Metalworking processes of many kinds are used in the fabrication of metal goods, and include operations in which the metal is formed by ductile flow with no removal of metal. Examples of such metalworking operations include rolling, forging, molding, stamping, casting, ironing, drawing, and extruding. In one type of metalworking operation, the metal is preheated to at least about 800° C. so that it can be more easily formed into the desired shape.
In addition to such hot fabrication methods, cold working techniques are also practiced. Cold working involves the physical deformation of a metal article (e.g., a blank, slug, or preform) at a temperature well below the recrystallization temperature of the metal, and preferably within about 200° F. of room temperature. Typical cold working processes include cold extrusion, cold heading, and wire and tube pulling deformation operations.
In order to deform a metal article, all cold working processes involve contacting the metal article with at least one die to work the metal article. When a metal is passed through a die during cold working, the friction between the metal being worked and the die causes the generation of heat. The heat caused by work hardening during deformation also increases the temperature of the die surface and the metal article. The generated heat, in turn, can have detrimental effects on both the dies and on the metal articles. For example, the die can wear rapidly due to the generated heat. Also, the heat can generate impurities on the surface of the metal article.
Accordingly, it is desirable to reduce the friction by applying a lubricant to the metal surface and/or the die in order to avoid or minimize the generation of heat during cold working. In order for a lubricant to remain in contact with the metal during one or more cold working drawings, a coating (i.e., a lubricant carrier) is typically first adhered to the metal surface.
Numerous zinc phosphate coatings have been proposed in order to achieve this function as a lubricant carrier. It is well known to form metal phosphate coatings by the application of an aqueous acidic phosphatizing solution to metal surfaces. The surface of the metal reacts with the solution to form an integral layer of substantially insoluble crystalline phosphate on the surface of the metal. This crystalline layer increases the ability of the metal to retain a uniform film of lubricant over the entire surface. This lubricant reduces metal to metal contact in cold forming operations. In addition to serving as a lubricant carrier in preparation for cold working, some zinc phosphate coatings can improve corrosion resistance of the metal surface and improve the adhesion of paint to the metal surface.
For example, U.S. Pat. No. 5,588,989 to Vonk et al. discloses the use of an oxime accelerator in a zinc phosphate bath which may also contain fluoride ions, nitrate ions, and ions of various metals, such as nickel, cobalt, calcium, magnesium, manganese, and iron.
In addition, U.S. Pat. No. 5,603,818 to Brent et al. is directed to a process for treating metal parts to provide rust-inhibiting coatings in at least five discrete steps. The first step in this process is the use of an aqueous phosphatizing solution that is used to deposit a phosphate coating on a metal surface. Various inorganic phosphate solutions are suggested, including zinc phosphate. Chlorate ions are disclosed as an additive to the phosphate solution.
The zinc phosphate process however carries with it a number of disadvantages. It involves the use of a heavy metal, zinc, and thus is not ideal from an ecological perspective. The phosphate process also produces a ferric phosphate sludge, which requires periodic shutdown of the process for manual removal, with resulting costs to productivity and additional costs for the disposal of the waste such as hauling costs and expenses related to obtaining permits for the disposal. There is also required a rinsing step after the application of the phosphate treatment, to stop the reaction, and there results from this an ongoing waste treatment of the rinse water. A further rinse, usually employing lime or borax, is then performed in yet another tank. Beyond all of these inconveniences and expenses, there is typically an emission of nitrogen oxides from the treatment bath, requiring appropriate personnel and environmental protection measures.
Thus there would be a benefit to providing methods of imparting lubricity to ferrous metals, without the use of traditional zinc phosphate methods.